The present invention relates to thermally imageable elements for use in direct thermal imaging systems. Imaging methods of the invention utilize thermally induced catalytic transformation of image-forming chemistry within the elements to provide an image without the need for photosensitivity (that is the incorporation of any photosensitive component).
Thermal imaging is a process in which images are recorded by the use of imagewise modulated thermal energy. A review of thermal imaging is provided, for example, in Imaging Systems by Jacobson and Jacobson (Focal Press, 1976). In general, there are two types of thermal recording systems.
In one system the image is generated by thermally activated transfer of a heat absorbing material from a donor element to a receiver element, while the other general process involves thermal activation using chemical or physical modification of components of a single imaging element. Processes of the first type include thermal dye transfer systems in which a dye is thermally transferred from one element (the donor sheet) to a second layer (the receiver sheet) as described, for example in U.S. Pat. No. 4,621,271 (Brownstein) and U.S. Pat. No. 5,618,773 (Bailey et al). Such systems, while providing color images of high quality, suffer from the disadvantage of requiring two sheets and the associated printer hardware for such a physical transfer of dye between two sheets.
Systems of the second type are those in which the image is formed in the element that is imagewise exposed using heat. The discussion that follows relates to systems of the second type.
Thermal energy can be delivered in a number of ways, for example, by direct thermal contact or by absorption of electromagnetic radiation. Examples of useful radiant energy sources include infrared lasers, thermal print heads, and electron beam devices. Modulation of thermal energy can be by intensity or time or both. For example, a thermal print head comprising microscopic resistor elements is fed pulses of electrical energy that are converted into heat by the Joule heating effect. In a particularly useful embodiment, the pulses are of fixed voltage and duration and the thermal energy delivered is then controlled by the number of such pulses sent to the print head. Radiant energy can also be modulated directly by means of the energy source, for example the voltage applied to a solid state laser.
Direct imaging by thermally induced chemical change in a recording element usually involves an irreversible chemical reaction which takes place very rapidly at elevated temperatures (for example, above 100xc2x0 C.). At room temperature the reaction rate is orders of magnitude slower such that, effectively, the material is stable at the latter temperature. A particularly useful xe2x80x9cdry silverxe2x80x9d direct thermal imaging element uses an organic silver salt in combination with a reducing agent. In this system the chemical change induced by the application of thermal energy is the reduction of the transparent silver salt to a metallic silver image by the reducing agent incorporated in the coating formulation. Such thermographic elements, after imagewise thermal exposure, provide a final image without the need for any post-exposure solution processing.
In addition to the dry silver imaging elements, non-silver dry photothermographic imaging systems are also known. For example, it is known to produce tellurium images by disproportionation of tellurium dihalides, as illustrated U.S. Pat. No. 3,700,448 (Hillson et al). The images are formed in the presence of a processing liquid that promotes the disproportionation amplification reaction in the presence of catalytic amounts of photogenerated elemental tellurium (Te0). The tellurium dihalides, however, are dark in color causing poor image discrimination. Further, the tellurium dihalides are typically unstable in air and undergo light induced decomposition only when moistened with an organic solvent. Accordingly, the tellurium dihalides do not satisfy the needs of dry processing.
It is also known that certain tellurium (IV) compounds wherein the tellurium is bonded directly to one or more carbon atoms can be used in photothermographic imaging. In GB-A-1,405,628 certain tellurium compounds, wherein the tellurium is bonded directly to a carbon atom, are described as useful image forming materials in thermally developed systems. The process using these organotellurium (IV) compounds to form a tellurium image is a unit quantum photoreduction, that is the Te0 is formed in a stoichiometric reaction by reduction of the Te(IV) compound by the photogenerated organic reducing agent. This process lacks any amplification and is, therefore, inherently slow in speed and, as a result, limited in usefulness.
An amplification step is an important factor in imaging systems having high speed. In such processes and elements, typically a redox reaction is catalyzed by a material that is generated in the exposure step. In the highest imaging speed materials, conventional wet processed silver halide photographic materials, high speeds are attributable to the following amplification process: exposure of photographic silver halide to light results in formation of small silver nuclei on the silver halide grain surfaces that catalyze the further reduction of silver halide in these exposed grains in a subsequent solution development employing a developing agent (a reducing agent) to give elemental silver in a high gain catalytic reaction.
Imaging materials have been described wherein a substance capable of darkening when heated is employed in the presence of a catalyst, such as described in U.S. Pat. No. 1,939,232 (Sheppard et al). This imaging material employs a compound such as silver oxalate to form an image and a compound such as tellurium dichloride as a catalyst. Thus, this system is quite different from the conventional photothermographic systems described above that rely on silver or a non-silver material, such as Te0 to provide image density after an imagewise light exposure to produce a developable latent image, and a subsequent uniform heating of the entire imaged element to produce the final visible image.
Materials are also known in the imaging art in which metal nuclei are used to initiate physical development processes. For example, processes in which such catalytic metal nuclei are generated by a light exposure step and subsequently amplified by solution physical are well know in the art, as illustrated in U.S. Pat. No. 3,719,490 (Yudelson et al).
Thermally processed non-silver photographic processes that incorporate redox amplification have also been described in the art. For example, imaging elements containing a photosensitive catalyst precursor, along with a physical development element comprising a Te(II) or Te(IV) compound, incorporated in a polymeric matrix with an organic reducing agent, are exposed to a suitable light source and then thermally developed to give a dense, black image of elemental tellurium. Such elements are referred to as xe2x80x9cphotothermographicxe2x80x9d that is an initial exposure step produces nuclei which act as a catalyst for the chemical reduction of the Te(II) or Te(IV) compound to Te0 by an organic reductant upon subsequent thermal development of the exposed element. Thus, a small amount of invisible photoproduct (the xe2x80x9clatent imagexe2x80x9d) is converted into a high density image by utilizing its catalytic property to initiate a redox reaction with a high amplification factor. Thermally processed photothermographic elements of this type have been described in U.S. Pat. No. 4,097,281 (Gardner et al) and U.S. Pat. No. 4,152,155 (Lelental et al).
In contrast to the above imaging processes involving light exposures, there has been a continuing need to provide improved thermographic compositions and processes in which an element can be thermally addressed to give directly an image without the need for an initial light exposure step. The use of so-called dry silver elements for this purpose is well known in the art. Such elements comprise a redox couple of a light stable silver salt, such as silver behenate, and an organic reducing agent incorporated in a polymeric matrix with various coating addenda, as described, for example, in U.S. Pat. No. 5,587,350 (Horsten et al) and U.S. Pat. No. 5,629,130 (Leenders et al).
Such thermographic silver systems generally incorporate a high coverage of the silver salt to produce a useful image density (typically from 40 to 85 mg/dm2). In addition to the cost associated with the use of such silver compounds, these systems require a time consuming and expensive manufacturing process involving dispersing of the water insoluble silver behenate particles to give a material which can produce good quality coatings. Therefore, a need exists for silver or non-silver thermographic systems employing a catalytic thermal development process with a high level of amplification and lower energy requirements. In addition, a need exists for system elements employing an image forming composition that can be readily dissolved in a polymer solution and conveniently coated on a suitable support, thus reducing the cost and inconveniences of manufacture noted above for conventional colloidal dispersion-based systems.
In its broadest sense, the present invention provides a thermally imageable element comprising a support having thereon one or more layers, the element further comprising:
image-forming chemistry that comprises i) image precursor chemistry, and ii) a catalyst or catalyst precursor that upon imagewise heating is capable of promoting thermally induced image formation with the image precursor chemistry, the i) and ii) components being in reactive association and uniformly dispersed or dissolved within a binder in the one or more layers,
the element capable of being thermally addressed to provide a visible image as a result of thermally induced catalytic transformation of the image-forming chemistry.
In addition, this invention provides a process of forming an image comprising imagewise thermally addressing the thermally imageable element described above at a temperature of at least 75xc2x0 C.
In a preferred embodiment, this invention is directed to a non-photosensitive thermally addressable imaging element comprised of a support having thereon in reactive association:
i) an oxidation-reduction image-forming combination (i.e. image precursor chemistry) comprising:
a) a reducing agent, and
b) an oxidizing agent to produce an elemental metal, metal compound or dye on reaction with the reducing agent, the reducing agent and oxidizing agent being separate compounds or components of the same compound,
ii) a catalyst or catalyst precursor capable of promoting the oxidation-reduction reaction of a) and b) on heating, and
iii) a binder,
wherein the oxidizing agent is comprised of a leuco dye or a selenium, tellurium, bismuth, copper or nickel compound that is a.
In still another embodiment, this invention is directed to a process of forming an image in the non-photosensitive thermally addressable imaging element described above comprising imagewise thermally addressing the element to a temperature of at least 80xc2x0 C.
The present invention provides a means for using a catalytic transformation during thermal imaging of the thermally addressable elements. In all embodiments, the image-forming chemistry (components i and ii) needed for providing an image is uniformly dispersed or dissolved within one or more layers of the element as opposed to being disposed in a predetermined pattern.
The present invention offers the capability of avoiding the disadvantages of the dry thermographic imaging systems discussed above. Specifically, the present invention does not require photosensitive silver compounds for imaging and also achieves image amplification. The elements of the invention can be dissolved in and coated from a polymer solution, and are thus more convenient to manufacture than the non-catalytic silver behenate type dry silver thermographic systems that are commonly used.
In the preferred embodiments, the catalytic transformation promotes an oxidation-reduction reaction in the uniformly dispersed image precursor chemistry to provide the image. This is preferably accomplished in a single step wherein a uniformly dispersed catalyst initiates the oxidation-reduction reaction. Alternatively, a uniformly dispersed, thermally-sensitive xe2x80x9ccatalyst precursorxe2x80x9d can be transformed during application of thermal energy into the catalyst that then induces the desired oxidation-reduction reaction.
In still other embodiments, the uniformly dispersed catalyst or catalyst precursor can induce other chemical or physical changes of the image precursor chemistry to provide the desired image. For example, in response to thermal energy, the catalyst or catalyst precursor can react with the image precursor chemistry to cause a change in pH or hydrophilicity or to bring about polymerization or isomerization reactions. Those changes in turn provide an image.
Still again, application of thermal energy can cause a physical change of some type, such as the breaking of barriers that normally keep the image precursor chemistry separated from the catalyst or catalyst precursor prior to imaging. For example, either the image precursor chemistry or catalyst (or catalyst precursor) can be encapsulated, and the vesicular or microcapsular walls can be broken during heating to allow the desired chemical reactions to occur. In still another embodiment, heating can allow intermixing of the components of the image-forming chemistry that were separated by a barrier layer prior to thermal imaging. Other means of using these features of the catalytic image-forming chemistry of this invention would be readily apparent to one skilled in the art in view of the teaching and references noted below.
All of these various embodiments demonstrate the advantages of the present invention wherein catalytic thermal imaging can be achieved with lowered activation energies, compared to prior art non-catalytic thermal chemical systems such as thermographic silver systems. The incorporation of such a catalytic imaging forming process allows imaging in shorter imaging times and/or at lower temperatures compared to conventional thermal imaging (for example non-catalytic systems). Moreover, they provide a variety of means for achieving the desired thermally-induced images from a variety of imaging devices and systems, thereby providing greater flexibility in thermal imaging for the industry. In addition, thermal addressing the elements of this invention can be achieved either with direct thermal contact such as by use of a thermal print head, or by irradiation such as by addressing the imaging element selectively using an infrared laser.
Lastly, exposure to actinic radiation such as visible or UV light is not required for imaging as is the case in some thermal xe2x80x9cdevelopmentxe2x80x9d systems for example as described in U.S. Pat. No. 4,152,155 of Lelental et al. The noted patent describes materials that are thermally developed after a separate step for latent image formation. In contrast, the materials of the present invention are thermally imaged (using thermal catalysis) and developed in a single step. Thus, the present invention requires no pre- or post-treatment steps besides the single thermal imaging step.